1. Field of the Invention
The present invention relates generally to electrical circuits for supplying electrical energy for energizing cold-cathode fluorescent lamps ("CCFLs") and, more particularly, to electrical circuits for simultaneously energizing at least a pair of CCFLs with electrical energy supplied from a single transformer.
2. Description of the Prior Art
CCFLs are used extensively to provide backlighting for passive displays, particularly for backlighting liquid crystal displays ("LCDs") used with digital computers. However, traditionally such backlighting applications have required only a single CCFL which is generally energized using an electrical circuit such as that illustrated in FIG. 1. As depicted in FIG. 1, for a typical backlighting application an alternating current ("AC") energy source 22 applies electrical energy at a frequency of approximately 20 to 100 kilohertz ("KHz"), and at a comparatively low voltage, e.g. 3.0 to 25.0 volts, across a primary winding 24 of a step-up transformer 26. The transformer 26 also includes a secondary winding 28 that has many more turns than that of the primary winding 24, e.g. 100 times more turns. Thus, the transformer 26 increases the comparatively low voltage AC applied across the primary winding 24 to an approximately 300 to 2,500 volt AC voltage across the secondary winding 28.
To energize operation of a single CCFL 32, the CCFL 32 is connected in series both with the secondary winding 28 of the transformer 26 and with an isolation capacitor 34. The isolation capacitor 34 provides both direct current ("DC") blocking and electrical isolation between the secondary winding 28 and the CCFL 32. For conventional CCFLs 32 used for backlighting computer LCDs, the isolation capacitor 34 usually has a capacitance of approximately 10 to 68 pico-farads ("pf").
When the CCFL 32 is off, i.e. not emitting any light, initially the AC voltage applied across the CCFL 32 rises to a break-down voltage of approximately 1,000 to 1,600 volts. Before the voltage across the CCFL 32 reaches the break-down voltage, the CCFL 32 is in a non-conductive state, i.e. essentially no electrical current flows through the CCFL 32, and only leakage electrical currents flow through the secondary winding 28 and the isolation capacitor 34. When the CCFL 32 breaks-down, enters a conductive state in which an appreciable electrical current flows through the CCFL 32, and the CCFL 32 begins emitting light; the voltage across the CCFL 32 drops to a sustaining voltage of approximately 350 to 600 volts. This phenomenon in which the voltage across the CCFL 32 drops to the sustaining voltage when the CCFL 32 becomes electrically conductive is frequently referred to as negative-impedance. After the CCFL 32 becomes conductive and begins emitting light, the voltage across the isolation capacitor 34 remains essentially constant during an interval which lasts until the AC voltage across the secondary winding 28 drops below the sustaining voltage for some interval of time. Because comparatively high frequency AC energy is supplied to the primary winding 24 of the transformer 26, returning the CCFL 32 to the non-conductive state requires reducing the voltage across the lamp below the sustaining voltage for an interval of time that is much longer than one cycle of the AC voltage across the CCFL 32. When the AC voltage across the secondary winding 28 drops below the sustaining voltage for a sufficiently long interval of time, the CCFL 32 again becomes non-conductive and stops emitting light. During each cycle of the AC voltage after break-down initially occurs, the CCFL 32 conducts electricity twice with electrical current flowing through the CCFL 32 first in one direction during a first sustaining voltage interval, and then in the opposite direction during a second sustaining voltage interval.
Recently, computer displays have begun using larger area LCDs that require using at least two (2) CCFLs 32a and 32b for proper backlighting. FIG. 2 depicts one circuit that may be used for energizing operation of two (2) CCFLs 32a and 32b in which the two (2) CCFLs 32a and 32b are connected in parallel across the secondary winding 28 of the transformer 26 respectively by identical, individual isolation capacitors 34. However, the negative-impedance characteristics of the CCFLs 32a and 32b implies that the lower, sustaining voltage across the CCFLs 32a and 32b occurs during intervals in each AC cycle in which a significant electrical current flows through the CCFLs 32a and 32b. Thus, selection of the isolation capacitors 34a and 34b becomes very critical in achieving proper operation of the CCFLs 32a and 32b in the electrical circuit depicted in FIG. 2.
The amount of light produced by each of the CCFLs 32a and 32b depicted in FIG. 2 depends strongly upon the electrical current flowing through the respective CCFLs 32a and 32b. The more electrical current flowing through the CCFLs 32a and 32b, the brighter the light emitted. As set forth below electrical current flows through each of the parallel connected isolation capacitors 34a and 34b and CCFLs 32a and 32b in accordance with Kirchoff's voltage law. EQU V.sub.P =V.sub.C.sbsb.1 +I.sub.1 Z.sub.1 =V.sub.C.sbsb.2 +I.sub.2 Z.sub.2
where
V.sub.P is the voltage across the secondary winding 28. PA1 V.sub.Ci is the voltage across capacitor "i." PA1 I.sub.i is the current flowing through the CCFL 32.sub.i PA1 Z.sub.i is the impedance of the CCFL 32.sub.i PA1 1. the size of the transformer 26 increases approximately 1.8 to 2.0 times; PA1 2. it is difficult to design isolation capacitors 34a and 34b to match with the CCFLs 32a and 32b under all operating conditions; and PA1 3. the CCFLs 32a and 32b exhibit non-uniform brightness due to current-hogging which also reduces the life of the CCFLs 32a and 32b, and the reliability and performance of the electronic system that includes the CCFLs 32a and 32b. PA1 1. the transformer 26 is approximately 1.8 to 2.0 times larger; PA1 2. the voltage rating of the isolation capacitor 34 depicted in FIG. 3 must be two (2) times greater than for the isolation capacitor 34 depicted in FIGS. 1 and 2; and PA1 3. parasitics such as stray capacitances and wire inductance between the CCFLs 32a and 32b make equalizing the current flowing through each of the CCFLs 32a and 32b difficult.
The " " over each of the parameters represents the phase of AC flowing through the electrical component described by that parameter.
It is readily apparent from the circuit depicted in FIG. 1 and from the preceding equation that either of the two (2) CCFLs 32a and 32b can "hog" substantially all the electrical current supplied by the secondary winding 28 of the transformer 26. If one (1) of the two (2) CCFLs 32a and 32b hogs all the current, the life of that particular CCFL 32 will be shortened which correspondingly shortens the life of the pair of CCFLs 32a and 32b. Correspondingly, if one of the CCFLs 32a and 32b hogs all the electrical current supplied by the secondary winding 28, then that particular CCFL 32 will be brighter than the other CCFL 32, and backlighting of a LCD will not be uniform.
Another disadvantage of the preceding circuit is that the wire used for the secondary winding 28 of the transformer 26 depicted in FIG. 2 must be significantly larger than that used for the secondary winding 28 depicted in FIG. 1. Normally, the full-rated electrical current for each of the CCFLs 32a and 32b is approximately 5 milliamperes ("mA") root-mean-square ("rms"). Therefore, the rms electrical current flowing through the secondary winding 28 of the transformer 26 depicted in FIG. 2 is approximately 10 mA rms. To maintain electrical efficiency of the transformer 26, doubling the electrical current flowing through the secondary winding 28 requires doubling the size of the wire used for the secondary winding 28. Doubling the size of the wire used for the secondary winding 28 approximately doubles the size of the transformer 26.
In summary then, the circuit depicted in FIG. 2 for energizing the operation of two CCFLs 32a and 32b, which is a conventional extension of the circuit depicted in FIG. 1, exhibits the disadvantages that:
FIG. 3 depicts another circuit that may be used for energizing operation of two (2) CCFLs 32a and 32b in which the two (2) CCFLs 32a and 32b are connected in series with a single isolation capacitor 34 across the secondary winding 28 of the transformer 26. In comparison with the circuit depicted in FIG. 2, the circuit in FIG. 3 ensures that substantially the same electrical current flows through both of the CCFLs 32a and 32b. Therefore when incorporated into the circuit depicted in FIG. 3, both of the CCFLs 32a and 32b emit approximately the same amount of light, and operation in the circuit depicted in FIG. 2 does not significantly reduce the life of either of the CCFLs 32a and 32b.
However, to ensure that the voltage applied to the series connected CCFLs 32a and 32b exceeds twice the break-down voltage of the individual CCFLs 32a and 32b, the output voltage that the secondary winding 28 may apply across the series connected isolation capacitor 34 and CCFLs 32a and 32b must be twice the output voltage applied by the secondary winding 28 of the transformer 26 depicted in either FIGS. 1 or 2. Consequently, in general the secondary winding 28 of the transformer 26 depicted in FIG. 3 must have twice as many turns as the secondary winding 28 of the transformer 26 depicted either in FIG. 1 or FIG. 2 even though the size of the wire used for the secondary winding 28 remains the same. However, doubling the voltage produced by the transformer 26 not only increases the number of turns required for the secondary winding 28, but also mandates increased electrical insulation to prevent break-down and arcing at the higher peak voltage.
In summary then, the circuit depicted in FIG. 3 for energizing the operation of two CCFLs 32a and 32b, which is also a conventional extension of the circuit depicted in FIG. 1, exhibits the disadvantages that: